This invention relates generally to amplifier circuitry and more particularly to base line restoration circuitry for amplifiers. More particularly still, the invention relates to an improved fast-recovery circuit for amplifiers.
In a variety of applications employing amplifiers, the input signal, and more importantly the output signal, may develop some degree of DC or low frequency offset which may tend to distort the informational content of any desired signal super-imposed thereon and/or may further cause a display thereof to be driven off scale. Therefore it is desirable to remove or minimize this offset such that the output signal reflects only a nonoffset representation of the desired signal applied to the input. Various offset correction or base line restoration circuits have been developed for cancelling or substantially eliminating offset appearing in the output signal. One technique used to accomplish this end is the employment of a negative feedback loop connected from the output of the amplifier through an integrator to the amplifier input. The integrator is substantially nonresponsive to signals having frequencies much above DC and accordingly only provides feedback of a correction or cancelling signal corresponding with the DC or very low frequency offset value. This technique minimizes attenuation of the desired signal.
In addition to very low frequency or DC offset values, other large signals such as pacemaker and/or defibrillation pulses appearing at the input of an ECG amplifier may cause an overload condition which so affects the RC integrating circuit of the feedback loop that restoration of the normal output base line may be delayed for a considerable time. During that time, a display provided by the amplifier output may be entirely off scale. To at least partially remedy this condition, speed-up or fast-recovery circuits have been used in conjunction with the integrator to change its time constant whenever the amplifier output exceeds some threshold value. This enables the output to return to operation within its voltage limits in a more rapid manner. Most of these fast-recovery circuits have employed a string of diodes connected in parallel with all or a portion of the resistance in the integrator's RC network. The accumulated voltage drops of the several diodes were utilized to define the threshold above which amplifier output levels would cause conduction through the diodes and bypass the resistance. The reduced RC time constant enables the integrator to respond more rapidly to any overload, such as result from sudden changes in electrolytic cell voltage of the patient monitored, from pacemakers, and defibrillators, to return the signal to the amplifier's linear output range. A drawback to this technique does exist, however, and has been termed by some as the "bump-bump" phenomenon. More specifically, when a high-voltage pulse or excursion does occur and the diodes conduct, a relatively large charge is rapidly applied to a capacitance connected to the integrator. Then, when the high-voltage pulse has passed and the diodes cease conducting, that large charge remains on the capacitance and the relatively large resistance of the RC time constant has similarly been returned to the circuit. This results in the base line of the amplifier's output being offset in the opposite direction of the pulse excursion, which offset is only slowly nullified because the integrator has now returned to its DC or low frequency response mode.
Accordingly, it is a principal object of the present invention to provide an improved fast-recovery circuit for an amplifier, such as used in ECG signal processing. Included within this object is the provision of a fast-recovery circuit which rapidly returns the amplifier output to operation within its voltage limits whenever excessively large amplitude very low frequency signals occur, yet substantially eliminates any resulting base line offset as the result of high amplitude, relatively higher frequency or short interval signals such as pacer pulses, large QRS complexes and the like.